Method of carbon sequestration

ABSTRACT

Described are methods, apparatus, and a system for robust and long-term sequestration of carbon. In particular, described is the sequestration of carbon as carbonates, using coccolithophorid algae grown using land-based aquaculture. Also described are methods of Ocean Thermal Energy Conversion (OTCE).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/998,764, filed Sep. 13, 2011, now U.S. Pat. No. 8,278,082, whichapplication is a national phase entry under 35 U.S.C. §371 ofInternational Patent Application PCT/GB2009/002568, filed Oct. 28, 2009,designating the United States of America and published in English asInternational Patent Publication WO2010/049687 A1 on May 6, 2010, whichclaims the benefit under Article 8 of the Patent Cooperation Treaty toGreat Britain Patent Application Serial No. 0819865.7, filed Oct. 29,2008, the disclosure of the entirety of each of which is herebyincorporated herein by this reference.

TECHNICAL FIELD

The disclosure relates to methods and apparatus for robust and long-termsequestration of carbon using natural carbon fixation. In particular,the disclosure relates to sequestration of carbon as carbonates, usingcoccolithophorid algae.

BACKGROUND

Excess global warming that is currently taking place is thought to belargely caused by human activity increasing the “greenhouse effect.”Since the industrial revolution, anthropogenic emissions have increasedthe amount of greenhouse gasses present in the atmosphere. Inparticular, combustion of fossil fuels has led to an increase in theatmospheric concentration of the greenhouse gas CO₂.

In order to mitigate the effects of global warming caused by atmosphericCO₂, attempts have been made to capture and sequester carbon. CO₂ can becaptured at point sources, such as power or cement plants, to prevent itfrom being released into the atmosphere or it can be removed from theatmosphere at remote sites with technologies that remove CO₂ directlyfrom the air.

Once captured, the CO₂ can be stored in a number of ways; for example,in deep geological formations, in deep ocean masses, in the form ofmineral carbonates or in the form of bio-char. In the case of deep oceanstorage, there is a risk of re-emission and of greatly increasing theproblem of ocean acidification, a problem that also stems from theexcess of carbon dioxide already in the atmosphere and oceans.Geological formations are currently considered the most promisingsequestration sites.¹ However, the use of limited geological sitesrequires transport of the CO₂ in pipelines, either as a gas or as asupercritical liquid. CO₂ storage in geological formations is,therefore, associated with further energy consumption to transport theCO₂ and to inject it into underground geological formations. Leakage ofthe stored carbon is also a major concern with both ocean and geologicalcarbon sequestration.

Mineral sequestration traps carbon in the form of solid carbonate metalsalts. One way to sequester carbon as carbonates is to use algae, inparticular, coccolithophorid algae, which are marine algae that formCaCO₃ coccoliths. These algae take up CO₂ from the atmosphere to formcoccoliths, thus removing CO₂ from the atmosphere and storing it inmineral form. Natural ocean-based coccolithophorid algal blooms are awell-documented method of carbon fixation in coccoliths as is evidencedby many limestone deposits worldwide. However, ocean blooms ofcoccolithophorid algae are unpredictable, and the algae cannot beharvested to ensure that the sequestered carbon is stored long-term toprevent its re-release back into the environment via remineralization ofthe algae after they complete their periodic and unpredictable, growthcycle. Natural blooms do not suffice to compensate for the increasedatmospheric CO₂ ² and cannot be controlled.

SUMMARY OF THE DISCLOSURE

The inventor hereof has concluded that many problems that are associatedwith the manipulation of ocean-based blooms of natural algae for carbonsequestration can be overcome by the use of land-based aquaculture ofcoccolithophorid and other algae. However, land-based aquaculturerequires large amounts of nutrient-rich seawater, and also requireslarge amounts of space for the aquaculture preserve.

Provided is a method of sequestration of CO₂ from the atmosphere intosolid form, the method comprising:

-   -   culturing coccolithophorid algae in seawater using land-based        aquaculture under conditions wherein atmospheric CO₂ is        converted by the algae into CaCO₃ and/or bio-char;    -   wherein the seawater in which the algae are cultured is the        discharge of an ocean thermal energy conversion (OTEC) process;        and    -   wherein the source water for the OTEC process is provided by i)        cold seawater and ii) seawater that has been warmed in        land-based greenhouses using solar energy.

The method makes use of an OTEC process that has been modified to makeit suitable for use in combination with large-scale land-basedaquaculture of coccolithophorid algae for carbon sequestration. OTEC isa method for generating electricity that utilizes the temperaturedifference that exists between deep and shallow waters. The use of OTECis currently limited to particular geographical areas where thetemperature difference between the warm surface water and the cold deepseawater is large, ideally at least 20° C. This temperature differenceonly really occurs in equatorial waters, defined as lying between 10° Nand 10° S, which are adequate.³ In these areas, coastal land use isintensive. In contrast, in areas where there is significant availabilityof coastal land that lies unused, the surface water is cool and, thus,there is an insufficient temperature differential for OTEC to functionefficiently.

The inventor hereof has realized that by heating seawater using solarenergy under controlled conditions, a sufficient temperature differencecan be established between this heated water and cooler, nutrient-richdeep seawater to allow OTEC to be used in regions where land isunutilized and/or available. This modified OTEC system can thus be usedin regions with vast areas of arid, desert or under-utilized ornon-productive coastal land (such as in the Gulf States, the CalifornianPeninsula in Mexico, Australia, Western, Northern and Southern Africaand Chile), which are ideal for large-scale land-based aquaculture.Because this land is commonly not used for economic benefit, theeconomics of this modified system become practical for carbonsequestration.

This solves several problems from which land-based aquaculture of marinealgae suffers, at least in the context of carbon sequestration. Ofcourse, pumping nutrient-rich seawater onto land requires large amountsof energy. Given that the purpose of these aquaculture preserves is toprovide a net carbon sink, any CO₂ produced in order to supply the waterfor the aquaculture must be more than offset within the CO₂ that issequestered by the algae. Thus, only renewable, or carbon neutral,sources of energy are suitable in this context. Examples of renewableenergy sources include solar energy, high altitude and ground level windpower, tidal, hydroelectric, and biomass fueled power generation. Othercarbon neutral, although not renewable energy sources, such as nuclearor geothermal power may also be suitable to generate the power necessaryto pump the large quantities of water for the aquaculture preserves.Both nuclear and geothermal power have the advantage of creating heatedwater that could be exploited by OTEC.

A much preferred source of energy for use in the method hereof is OceanThermal Energy Conversion (OTEC), for example, as described inreferences 3 and 4. One reason for this is that one of the by-productsof OTEC is an abundant supply of nutrient-rich deep seawater, which isgenerally considered to be a by-product of OTEC, and is referred toherein as OTEC discharge. This makes OTEC a suitable source of energy tocombine with aquaculture, as described in references 5 and 6 describingthe combination of sea-water wells extending through rock andaquaculture with power generation. The sea-water well concept is limitedto favorable geological conditions, for example, encountered in Hawaii.However, improvements in pipe-technology achieve the same benefits ofseawater wells without being geographically confined.

OTEC has been used in combination with open-water aquaculture for foodproducts, such as high-value farmable commercial species and algae forhuman consumption. However, these sites are limited in scale. Becausethe regions that are currently suited to the application of OTEC arealso regions in which the land surrounding the OTEC sites is valuableland, for example, for agricultural, habitational or recreationalpurposes, such as narrow coastal bays in Hawaii, OTEC cannot be used ona very large scale in these environments. For example, a 100 MW OTECpower plant in combination with land-based coccolithophorid algalaquaculture to be suitable to sequester large amounts of atmospheric CO₂requires an area of approximately 40 km² of aquaculture preserve orgreater. For these reasons, it would hitherto not have been consideredto combine OTEC with large-scale land-based aquaculture because the landavailable for the large aquaculture preserves has relevant alternativeeconomic utility. Indeed, the OTEC systems that are presently known inthe art are not suitable to be used as an energy source to supplyseawater for land-based aquaculture of coccolithophorid algae forlarge-scale carbon sequestration.

As well as extending the geographical range of OTEC, heating seawaterprior to use in OTEC is also advantageous because it increases thetemperature difference between the hot and cold water, thus increasingthe efficiency of the OTEC system.

A further advantage of the method of carbon sequestration describedherein, in which the surface water is heated before use in OTEC, is thatthe water is heated to a temperature that inactivates substantially allmicro-organisms, thus reducing bio-fouling of the OTEC system, the heatexchangers or the subsequent aquaculture preserves.

Carbon sequestration methods currently exist that are based on thestimulation of growth of algae in the ocean. However, the methods hereofare distinct from these in that they are carried out using land-basedaquaculture. Per the disclosure, algal growth takes place in anaquaculture preserve, which has several advantages.

In one embodiment of the method hereof, discharge from OTEC may becombined with cold seawater to provide water at the optimum temperaturefor coccolithophorid algae aquaculture. In an alternative embodiment,discharge from OTEC may be cooled by running it through a shaded areabefore it is seeded with algae. In this manner, the culture conditionssuch as water temperature can be adjusted such that the algae grow at amuch higher density than are commonly found in their naturalenvironment.

Nutrient availability can also be tightly controlled, both by varyingthe amount of nutrient-rich and cold seawater from depths of up to 1,000meters that enters the aquaculture preserve, or by adding exogenousnutrients to the aquaculture water. The step of adding exogenousnutrients to the aquaculture water forms a further aspect hereof.

A still further advantage is that algal growth is isolated from theocean ecosystem. This mitigates most, if not all, of the environmentalproblems associated with ocean-based algal blooms. In ocean-basedblooms, currents and atmospheric conditions can rapidly andunpredictably alter the growth environment. Predatory or competingorganisms can adversely affect the growth of the desired algae.Nutrients may be depleted or where nutrients are available at depth,there is insufficient sunlight to sustain the bloom. Containingmicroalgae in ocean-based aquaculture is impractical since thecontainment structures would have to be impermeable to microscopicorganisms on the outside of the aquaculture facility. Managing the algalbloom in situ also requires the effective harvesting of algae underdynamic open ocean conditions, which are not as stable and predictableas land-based aquaculture.

Furthermore, using land-based aquaculture, there can be no uncertaintyor ambiguity as to the amount of carbon that is sequestered. Thecoccolithophorid algae can be harvested as one aspect of the process,thus allowing the amount of sequestered carbon to be tangibly measuredand quantified. The step of harvesting the coccolithophorid algae thusforms a further aspect hereof.

Once the algae have been harvested, the culture water can be returned tothe ocean. After having been used for aquaculture of coccolithophoridalgae, the culture water generated in the method hereof will containsignificantly less dissolved CO₂ than ambient seawater and can,therefore, contribute to the reduction of ocean acidification. Ifnecessary, the culture water can be supplemented with dissolved calciumminerals to offset the effects of ocean acidification, which is itselfassociated with detrimental environmental effects. This is particularlyrelevant in the context of coastal coral that are particularly sensitiveto ocean acidification. Therefore, provided is a method of carbonsequestration as described herein, further comprising returning thewater to the sea after harvesting the coccolithophorid algae.

Also provided is a system for carbon sequestration, the systemcomprising:

-   -   a) means to heat seawater using solar power,    -   b) an OTEC system using heated surface seawater and cold deep        seawater; and    -   c) a land-based coccolithophorid algae aquaculture preserve        provided with water from the OTEC system of part b).

In one embodiment, the aquaculture preserve comprises “aquacultureponds.” As used herein, the term “aquaculture ponds” is intended todescribe man-made ponds that are long and narrow in shape and thatprovide a flow-through system for the aquaculture water. It is preferredthat the aquaculture ponds form a large surface area for growth of thealgae. Indeed, provided is an economically viable method of carbonsequestration in part because of the large scale of the aquaculturepreserve. In one embodiment, combining a 100 MW OTEC plant withgreenhouses and the aquaculture preserve gives a total surface area of30 km² to 100 km² or 50 km² to 100 km², depending on configuration and,in a particular embodiment, the surface area is about 65 km². Localgeography permitting, this arrangement of an OTEC plant, a greenhouseand aquaculture preserve can be significantly smaller, for example, fora 500 KW OTEC plant, the surface area could be less than approximately1.5 km². Similarly, larger OTEC power plants can be combined withaquaculture preserves that match their discharge. For example, a 500 MWOTEC plant, greenhouse, aquaculture assembly would require approximately300 km².

In one embodiment, the OTEC plant with greenhouses and aquaculturepreserve are a model system, and have a total surface area of 0.5 km² to1 km² or less. For many locations, the surface area required for thegreenhouses will be approximately half of the surface area required forthe aquaculture ponds. In the preferred embodiment near a cold surfacewater ocean outside of the conventional OTEC tropical ocean sites, thismeans approximately 20 km² for the greenhouses and 40 km² for theaquaculture ponds.

In any of the methods and systems described herein, the means to heatseawater using solar power may be a greenhouse. In one embodiment, thegreenhouse is made of a plastic material.

In any of the methods and systems described herein, the coccolithophoridalgae may preferably be a strain of Emiliana huxleyi. Other examplesinclude, but are not limited to, strains of Gephyrocapsa oceanica,Calcidiscus leptoporus, Coccolithus braarudii, Braarudosphaera bigelowi,and Syracosphaera pulchra.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1: Schematic diagram of the system for sequestering carbon.

DETAILED DESCRIPTION OF THE INVENTION

Methods of Carbon Sequestration

The method is based on growing large quantities of carbon sequesteringcoccolithophorid algae in nutrient-rich water, originating from deepcoastal seawater, on land. First, seawater is pumped from depth to sealevel. Deep seawater is more nutrient rich than surface water and has ahigher concentration of supersaturated Ca⁺⁺ ions in solution and, thus,is better suited for use in aquaculture. A portion of the water isheated, preferably using solar energy, for example, by passing the waterthrough large greenhouses for solar heating. If sufficiently nutrientrich, an alternative source of warm water can be surface seawater. Thisraises the temperature of the water to over 30° C., for example, to 35°C., 40° C., 45° C., 50° C., 55° C. or hotter. The remainder is notheated and, therefore, remains at a temperature of about 5° C. to 10°C., for example, 5° C., 6° C., 7° C., 8° C., 9° C., or 10° C. The heatedand unheated water is then used to generate electricity using OTEC. Theelectricity generated by this process can be used to process algae thathave been grown in the aquaculture preservers. The energy can also beused to pump the deep seawater to the surface, and/or to pump waterthrough additional aquaculture preserve, thereby increasing theaquaculture yield while reducing the total OTEC-generated poweravailable for resale.

The discharge water from the greenhouses and OTEC condensers can becombined in an aquaculture preserve to provide water at a temperaturesuitable for coccolithophorid algal growth. The water can be combined toprovide an optimum growth temperature. The optimum growth temperaturewill depend on the particular coccolithophorid algae species and istypically between 15° C. and 25° C., for example, 15° C., 16° C., 17°C., 18° C., 19° C., 20° C., 21° C., 22° C., 23° C., 24° C. or 25° C. orwarmer.

Coccolithophorid algae are cultured in the aquaculture preserve,generally for several days. In a preferred embodiment, the algae arecultured for four to seven days, but may be cultured for up to twentydays. In one embodiment, the algae are cultured for six days. Duringthis time of exponential growth, the algae sequester CO₂ as CaCO₃ in thecoccoliths or plates surrounding their cells. These calcite platesincrease in number according to environmental conditions or until thecells reach a steady-state growth phase and at which point thecoccoliths may increase further in number per cell.

After passing through the aquaculture preserve for several days, thealgae may be removed from the seawater. The seawater may then bereturned to the sea. The harvested algae can be dried, for example,sun-dried. The CO₂ taken up during the aquaculture process is stored inthe dried algae as CaCO₃ and dried biomass.

Sun-drying the biomass for long-term stock-piling near the aquaculturepreserve is the simplest option for carbon storage. It requires theleast infrastructure and investment and takes advantage of the totalcarbon captured in the biomass, and not only that captured in thecoccoliths. Sun-dried biomass can be readily quantified and the carboncontent can be demonstrably verified. Thus, the methods hereof are,therefore, suitable for the highest grade carbon offset schemes, as theamount of carbon sequestration is physically available for analysis andquantification.

In a separate embodiment, the coccoliths and biomass are separated andprocessed independently of each other. The coccoliths are inert and canbe readily stockpiled. The total volume of dried coccoliths to sequester1 billion tons of CO₂ in the form of CaCO₃ coccoliths is estimated at2,611 kg/m³ is 869,400 m³ sequestered per year or a volume of 20 metershigh by 200 meters wide by 200 meters long. Given that this isdistributed over many sites, for example, 1,000 aquaculture preserves inseveral countries, this does not represent a significant disposal orstorage burden for any one site. In one embodiment, the space requiredfor disposal is 10×10×1.75 meters of CaCO₃ per algal aquaculture pondper year.

In an alternative embodiment, the harvested algae can be fragmented intomultiple components. Processing of the harvested algae at a basic pHenables the separation of the coccoliths from the remaining organicbiomass. Once extracted, the organic biomass can be used for theproduction of bio-fuel, biological oils, fatty acids and other materialsthat can be used for the generation of, for example, animal feedstock.In a further embodiment, the remnants or bio-char from such furtherprocessing could be used to enrich the soil of, or to create fertilizerfor, local agriculture.

The additional processing is optional and can be performed based on thedesirability of creating additional products from the organic biomass,which would reduce the overall yield of the sequestered carbon althoughrenewable resources created from the biomass would still qualify forcarbon offset payments. Energy required for additional processing can besupplied from OTEC.

Coccolithophorid Algae

Coccolithophorid algae are single-celled algae belonging to the divisionhaptophytes. A model coccolithophorid alga for use in the invention isEmiliana huxleyi, or a variant thereof. E. huxleyi naturally blooms inwaters between 15° C. and 20° C. and higher. Unlike other algae, itsgrowth does not become inhibited by high levels of sunlight. Duringexponential growth, each cell produces approximately twenty CaCO₃ platesor coccoliths. These coccoliths, therefore, sequester carbon in mineralform and, therefore, sequester carbon in a robust manner.Coccolithophorid blooms often occur in relatively calm water, soproviding another advantage in the context hereof relative to otheralgae. Because coccolithophorid algae can grow in calm water, there islittle or no need for agitation of the aquaculture water beyond thatprovided by the movement of the water through the aquaculture ponds.

A further advantage of E. huxleyi, in particular, is its rapid growth.E. huxleyi grows rapidly and after four to seven days of land-basedaquaculture, it will achieve as many as ten doublings or up to athousand-fold increase in biomass. Under ideal culture conditions, E.huxleyi is capable of up to two doubling per day.⁷

By comparison, in the open ocean, coccolithophorid algae growth islimited by sub-optimal temperatures, limited nutrient availability,aging of the bloom and other biological and environmental factors.

In the land-based aquaculture hereof and, in particular, in theaquaculture ponds that are a preferred feature of the described system,nutrients including phosphate, nitrate and calcium can be added in acontrolled manner to ensure that the coccolithophorid algae grow at ahigher density than that which can be achieved in ocean-basedaquaculture. Furthermore, maximum exposure to light can be ensured, suchthat no biomass is lost due to sinking out of optimal light zones. Inaddition, biological contamination can be minimized by the pre-treatmentof the seawater in the greenhouses before it is used for aquaculture.

Another advantage of E. huxleyi is that as the algae mature, they beginreleasing long extra-cellular organic molecules called TransparentExopolymer Particles (TEPs).⁸ Together with coagulants and flocculants,these large molecules dramatically increase aggregation of the algae.

Flocculants and coagulants used in wastewater treatment, such as alumand polymeric flocculants, can rapidly accelerate the formation or“marine snow” as mature algae aggregate. These aggregates rapidly settleand can be easily harvested, thus reducing the cost of harvesting thealgae and extracting biomass from the seawater. In an alternativeembodiment, the aggregates can be rapidly floated to the surface usingfine bubble aeration. This can be accomplished by placing long rows offine bubble aerators along the bottom of the final stretch of theaquaculture ponds to create rows of bubble curtains that carry the“marine snow” to the surface for harvesting by mechanical skimmers. Asthe aggregated algae accumulates, the aggregates can be removed usingautomated systems skimmers directly from the surface of the aquaculturewater creating an algal slurry at approximately ten ten-foldconcentration of the algae.

An alternative harvesting technique is dissolved air floatation toconcentrate the algae on the surface of the aquaculture water for simpleskimming, as is commonly employed in wastewater treatment.⁹ Thesewell-established industrial and wastewater treatment techniques canpurify very large volumes of water. In another alternative, theaquaculture water can be passed through circular centrally drivenclarifiers (for example, suction clarifiers) to harvest the algae fromthe aquaculture water. The harvested aggregated algal sludge stillretains a significant quantity of seawater. This sludge can be furtherthickened in deep cone thickeners or clarifiers. To significantlyaccelerate this thickening process, cationic flocculants can be used toclump the algae into clumps effecting another five to fifteen-foldconcentration. This algal paste can then be transported on conveyorbelts to drying fields that are also lined with geofabrics orgeomembranes to dry in the ambient desert air where the aggregated algaeare dried, as is common, for example, with commercial crops.

The system does not have to be populated with E. huxleyi. Othercoccolithophorid algae can also be used. Aquaculture parameters can bechanged to optimize growth of any chosen coccolithophorid or otherdesirable algae.

After the algae have been harvested, the water can be treated, forexample, by being passed through the greenhouses again to heatinactivate any remaining cells. In this way, the potentially negativeimpact of the aquaculture water being discharged into local seawater canbe eliminated. Alternatively, the used seawater can be pumped to a depthwhere it bypasses the local surface seawater entirely.

Carbon Fixation by Coccolithophorid Algae

As a result of the differences in molecular weight of CO₂ and CaCO₃ (CO₂has a molecular weight of 44 MW; CaCO₃ has a molecular weight of 100MW), to grow sufficient coccolithophorid calcite plates to sequester onebillion tons of CO₂, 2.27 billion tons of coccoliths have to be grown.Since a single coccolith (one of about 20 to 30 plates duringexponential growth) of an Emiliana huxleyi cell¹⁰ typically weighs 18 to26 pg, each coccolithophorid cell can sequester approximately 230 pg ofCO₂.

Under unmodified natural growth conditions in unmodified surfaceseawater, i.e., ocean-based, coccolithophorid blooms have a typicaldensity of 3×10⁸ cells/liter.¹¹ This equates to the creation of about0.156 gr/calcite per liter in the form of coccoliths. This is well belowthe potentially maximum limit of available ˜0.95 gr of CaCO₃ sequesteredper liter of seawater as dictated by the availability of naturallyavailable dissolved calcium while preserving the stoichiometry ofdissolved calcium in seawater. By comparison, conditioned seawaterenriched with nutrients can sustain much higher growth rates, reaching amuch higher cell density of about 1×10¹⁰ cells/liter⁷ in K media¹² or inF/50 media.¹³ At 1×10¹⁰ cells/liter, this equates to 5.2 pg×10¹² calcitefixed per liter of growth medium. At these optimal high-growth densitiesin nutrient-enriched seawater, up to 5.2 gr of calcite can be fixed perliter. However, achieving this very high growth density and coccolithyield would require large-scale addition of nutrients and minerals.

Another method by which the coccolith yield could be increased would bethrough increased residence time of the algae in the ponds to supportgrowth at high-cell densities in late-stage growth (not exponentialgrowth). At this point in the cell cycle, a larger number of coccolithsare formed per cell. However, late stage, very high density culture ofcoccolithophorid algae would be much more difficult to control and theresidence time of the algae would have to be significantly increased,thereby dramatically increasing the size of the algal ponds. For thisreason, a preferred embodiment is planned that maximizes the yield ofalgae in exponential growth in the algal ponds and harvests these cellsat the point when they have between 20 to 30 coccoliths per cell.

It is envisaged that the algae will be grown at approximately 3.0 to3.5×10⁹ cells/liter in the invention, reaching an average of 1.5 gcalcite formed per liter of nutrient and mineral enriched seawater. Atthis density, much of the calcium and a significant proportion of themagnesium present in the seawater will also be sequestered in the algalbiomass.

Aquaculture

The aquaculture takes place in an aquaculture preserve on land. In apreferred embodiment, the aquaculture takes place in continuouslyflowing aquaculture ponds or raceway ponds. Flowing aquaculture pondsare continuously agitated habitats for algae. They are preferablysignificant in size, and open-air. In these aquaculture ponds, the algaeare exposed to sunlight, and the seawater is exposed to atmospheric CO₂while providing a controlled growth environment for the algae.

One advantage of aquaculture ponds is that they provide favorable growthenvironment for coccolithophorid algae by ensuring constant turnover ofthe seawater, preventing settling or stratification of the algae andensuring maximum air exchange. Air exchange is doubly significant,because the algal growth rate will rapidly deplete the inorganiccarbonate (HCO₃ ⁻) naturally occurring in the seawater. This will bereplenished from atmospheric CO₂ to sustain algal growth. Because of theturnover of the seawater in aquaculture ponds and their preferablyshallow cross section, all of the aquaculture seawater will befrequently exposed to the atmosphere to absorb CO₂.

In the preferred embodiment, the seawater in the aquaculture ponds isfed and completely replaced with fresh seawater from the greenhouses andOTEC condensers, and the entire volume of the aquaculture pond isreplaced, preferably every four to seven days, given an aquaculture pondwith a cross-section of approximately 100 meters and a depth ofapproximately 3 meters. The depth of the aquaculture ponds can be variedaccording to need to ensure that all the algae is optimally irradiatedwith sunlight. For example, aquaculture pond depth could be 0.02, 0.1,0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 or more meters. In thepreferred embodiment, this results in a flow rate of approximately 2.4meters per minute during daylight hours. At this flow rate and with therepeat turnover as the seawater flows around the bends of theaquaculture, ensuring frequent seawater turn-over, this preferredembodiment supports the exponential growth phase of the coccolithophoridalgae. The flow rate through the aquaculture ponds can be varied fromless than 0.5 meter per minute to greater than 10 meters per minute toadjust the residence time of the algae and maximize the capacity fortheir growth and carbon sequestration. For example, aquaculture pondflow rates could be 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 or more meters perminute.

The aquaculture ponds may be seeded with algae directly, or suppliedwith algae from seeding ponds. Seeding ponds are ponds that arepositioned in proximity to the aquaculture ponds and are used tomaintain the supply of algae for the inoculation of the unpopulatedseawater coming out of the OTEC greenhouse combination. These seedingponds are run at a lower throughput rate and, instead of beingharvested, are used to inoculate the aquaculture ponds with a largevolume of densely growing algae. In a preferred embodiment, about 5% to12%, for example, 6%, 7%, 8%, 9%, 10%, or 11%, of the water entering theaquaculture ponds on a daily basis is from these seeding ponds. In thisembodiment, one seeding pond can continuously support 10 to 12aquaculture ponds with algae. Use of seeding ponds, in effect, extendsthe residence time of the algae under optimal growth conditions andavoids having aquaculture ponds seeded at low density, thereby having touse less land for the aquaculture ponds. In an alternative embodiment,there are two or three tiers of seeding ponds that are staggeredsequentially and have different depths. In this manner, theconcentration of algae can be matched to the depth of the pond, toensure that the transparency of the aquaculture water is optimized forthe growth rate of the algae. In this way, less densely populatedaquaculture ponds can be deeper. The advantage of this is the moreeconomical use of land.

System Components

Water Heating Means

To heat the seawater, large greenhouses, preferably situated close to acoastline, will be used. In a preferred embodiment, to support a 100 MWOTEC plant, twenty greenhouses will each cover an area of between about25,000 m² to about 100,000 m². As a person skilled in the art willappreciate, a smaller OTEC plant will require greenhouses with a smallersurface area, and a larger OTEC plant will require greenhouses with alarger surface area. In one embodiment, the OTEC plant is a 500 KWplant, and the greenhouses each have a surface area of between about1,250 m² to about 5,000 m². In a further embodiment, a 500 MW OTEC plantis provided with greenhouses each having a surface area of about 125,000m² to 500,000 m².

The surface under these greenhouses will preferably be lined with blackimpermeable plastic linings or geomembranes, similar to those that areused in landfills. This will assist in absorbing and transferring solarradiation to a free-flowing layer of seawater approximately 10 cm deepdisposed across the length and width of each greenhouse. The depth ofwater in the greenhouses can range from less than 1 cm to 500 cm, forexample, 5, 10, 20, 50, 100, 200, or 500 cm. In areas requiringsignificant solar heating of the seawater, in a preferred embodiment,the greenhouses are each about 250 m to about 1,000 m long and about 250m to about 1,000 m wide and 3 m to about 5 m high.

In a preferred embodiment, the greenhouses are constructed from aplastic material. In a specific embodiment, the greenhouses areconstructed from double layered improved polyethylene (PE) with 180 μmPE UV IR outer folia and 50 μm inner folia. In this embodiment, thegreenhouse spans will cover multiple gutter-connected tunnels ofapproximately 10 meters in width and approximately 3 to 4 meters inheight.¹⁴ In a preferred embodiment, the inter-connected tunnels aresupported by plastic beams standing in the pool of freely flowingseawater on the ground of the greenhouses.

At the entrance to the greenhouses, the water is nutrient-rich seawaterfrom depth at a temperature of approximately 5° C. to 10° C., or surfaceseawater at a temperature of about 10° C. to 25° C. and as it flowstowards the exit of the greenhouse, it reaches temperatures of over 30°C., for example, 40° C., 45° C., 50° C., 55° C. or hotter. The flow rateof the water can be altered to control the length of time the waterspends in the greenhouse, and thereby control the temperature of thewater leaving the greenhouse. Typically, the flow rate will be between 2to 6 m/minute, preferably 4 m/minute.

Water vapor evaporating in the greenhouses as a consequence of the solarheating can be trapped inside the greenhouse. In one embodiment, thewater vapor may be actively moved through the greenhouses to a largecondenser, where it is cooled using seawater pumped from depth, which isat approximately 5° C.

In one aspect hereof, the freshwater condensate can be captured. It isestimated that in this manner, the water vapor created in a 12-hourperiod in one 50,000 m² greenhouse will, on average, produce at least600 tons (m³) of freshwater per day. Since there are up to 40greenhouses associated with a 100 MW plant in the preferred embodiment,24,000 tons of freshwater can be extracted from the water vapor in thegreenhouses alone, independent of the freshwater generated during OTECpower generation. Given the use of cold seawater already pumped for usein OTEC, it can also be used to cool the greenhouse water vapor, therebycreating freshwater condensate. It requires very little additionalenergy to harvest this additional freshwater resource from the watervapor created in the greenhouses. The greenhouse condensation step isperformed in the secondary cycle already used for the condensation ofwater vapor generated through the OTEC cycle.

Another advantage of the greenhouse pre-treatment step is that itenables the control of the conditions under which the algae are grown.For example, solar heating the water in the greenhouse can be used toinactivate competing or deleterious organisms that might otherwisecontaminate the coccolithophorid algae aquaculture or foul theequipment. Similarly, if there is unwanted biomass or particulates thatmight sediment in the aquaculture ponds, these contaminants can beeffectively settled and removed from the seawater as it is travellingthe length of the greenhouses.

In embodiments that involve the addition of nutrients to the seawater tooptimize conditions for coccolithophorid algal growth in aquaculture,the nutrients may be added to the warmed water in the greenhouse tofacilitate their dissolution.

By controlling the flow-rate of the water through the greenhouse, thetemperature of the water can be adjusted before it enters theaquaculture ponds to ensure temperature stability in the aquaculturepreserve, in spite of seasonal and environmental changes. Typically, theflow rate will be between 2 to 6 m/minute, preferably 4 m/minute. At aflow rate between 2 to 6 m/minute, the solar heating of the water willtypically be approximately 0.10° C. per minute to 0.5° C. per minute andhigher.

If, after mixing, the temperature of the blended cold and warm water isstill too high to support coccolithophorid growth, it can be cooled. Inone embodiment, the conditioned water is cooled by combining it withcold seawater pumped from depth. In a further embodiment, watertemperature entering the aquaculture ponds can be adjusted to theoptimum temperature for coccolithophorid growth by blendinggreenhouse-treated water and cold deep seawater. In another embodiment,the conditioned water is cooled by running it through a shaded sectionof the aquaculture pond before it is seeded with algae.

Energy Extraction

The seawater running through the greenhouse will have been heated andcarry large amounts of solar energy. This warmed seawater coming out ofthe greenhouses can be passed through an energy extraction system toextract the captured solar energy and convert it to electricity.

The primary purpose of extracting heat energy from the greenhouse-heatedseawater is to generate the energy necessary to move the large volumesof seawater that are required for the coccolithophorid algalaquaculture. Even though the pumps will only have to pump water over asmall distance and height, the pumps will move large quantities ofwater. In the preferred embodiment of this invention, with a 100 MW OTECplant, 400,000 tons/hour cold water and 400,000 tons/hour of surfaceseawater are pumped through the OTEC plant over a 24-hour period,equivalent to 19.2 million tons of water/day. The secondary purpose ofenergy extraction is, therefore, to create excess energy that can besold and/or drive complementary services and processes, for example, theharvesting of algae.

Due to the heat capacity of water, this energy extraction system isstable and based on the bulk heat trapped in seawater and, thus, will beunaffected by temporary fluctuations such as changes in weatherconditions like cloud cover.

In some embodiments, OTEC facilities can be used to generate electricityat night in the conventional mode, i.e., without greenhouse solar-heatedwater, to generate additional electricity on a 24-hour cycle. For thoseOTEC plants located in areas where the surface water is sufficiently hotto drive the OTEC energy production process without solar heating,energy flow is continuous, even after solar heating has stopped duringthe night. In other embodiments, the heated greenhouse water can bestored in large surge tanks to provide hot seawater after dark, therebyextending the operational hours of the heated-seawater OTEC system afterdark.

OTEC

Ocean Thermal Energy Conversion (OTEC) has been in development andpiloted extensively (see, Cuba 1930, Cote d'Ivoire 1956, Keahole Point1979, Nauru 1982-84, Hawaii 1993). OTEC is, therefore, awell-established technology that is used to generate electricity basedon the temperature difference between warm surface seawater and coolerdeep seawater. Conventional OTEC is designed to exploit the temperaturegradient of cool (4° C. to 7° C.) seawater and warm (20° C. to 28° C.)surface seawater to generate electricity.

OTEC works by pumping large volumes of warm, generally surface seawaterover a series of evaporators to create a driving gas that spinsturbines. The driving gas then is condensed in a heat exchanger withcold deep seawater.

There are four major variants of OTEC, any of which are suitable for usein the invention. The four major variants of OTEC are:

-   -   Closed cycle—where the warm water evaporates a driving gas, such        as ammonia, that is then condensed with cold seawater in a        closed system. The closed cycle uses a Rankine process with a        low pressure turbine.    -   Open cycle—the Claude process where the warm water is used to        create low pressure steam through flash evaporation in a low        pressure chamber. The low pressure steam is then used to drive        turbines and can then be condensed and harvested as freshwater.    -   Kalina process—which can be run in a closed cycle mode using a        mixed ammonia/water as the low boiling point liquid used to        drive the turbines more efficiently, thereby increasing the        energy yield of the OTEC process.    -   Hybrid process—where the warm and cold seawater used for power        is driven through a secondary loop used for the evaporation and        condensation of water vapor to create large amounts of        freshwater.

The particular OTEC system chosen will depend on the local geography andcan be configured as required to maximize the production of energy orfreshwater in addition to creating the algal biomass.

OTEC can be deployed in modules according to the volumes of watertreated. The solar heating of the seawater in the invention leads to ahigher relative temperature differential between the greenhouse-heatedwater and the coastal deep seawater than is found in conventional OTECsystems. This increased temperature differential increases theefficiency of the OTEC system. At a temperature differential of around40° C. between the deep ocean water and the solar heated water, a muchhigher energy extraction efficiency is expected.

In conventional OTEC systems, given the low temperature differential ofthe surface seawater to the cold deep water of approximately 15° C., theenergy extraction yield is theoretically limited to 6% to 7%.¹⁵ However,practically, the yield is closer to 2.16% for the Rankine process or4.5% for the Kalina process.¹⁶

In the modified OTEC systems used in the invention, the theoreticalthermodynamically maximum energy yield increases from a maximum 7% toabout 8.4% and, consequently, the operational energy yield can increaseapproximately from less than 4.5% to greater than 5%.

The increase in efficiency is due, in part, to the increased energycontent of the solar-heated seawater used in the invention, whichresults in a 240% increase in kWhr yield during daylight operating hoursand more than a 20% increase in extraction yield. A further increase inefficiency is due to the ease with which the heated seawater will createlow pressure steam through flash evaporation in the low pressure chamberof the hybrid open-cycle OTEC system.

In a further aspect, also provided is a method for increasing theefficiency of ocean thermal energy conversion (OTEC) wherein the sourcewater for the OTEC process is i) cold seawater and ii) seawater that hasbeen warmed in land-based greenhouses using solar energy. In a furtherembodiment of this aspect hereof, there is provided a system forgenerating energy using OTEC comprising a means for heating seawaterusing solar energy and an ocean thermal energy conversion (OTEC) systemadapted to use the heated surface water from step a) and cold seawater.

OTEC offers additional benefits when used in accordance with the methodhereof. For example, as described above, it can be used to generatefreshwater. It is estimated herein that up to 120,000 tons of freshwaterper day can be generated by a single 100 MW OTEC plant. Thisside-product may be especially beneficial in the arid locations wherethe invention will be exploited. Furthermore, the increased evaporationefficiency described above also significantly increases the yield offreshwater from the desalination component of the OTEC systems hereof.

Water Conditioning

By running the seawater through the greenhouse for solar heating, thesystem hereof pre-treats the seawater before it is used for aquaculture.In one embodiment, the water running through the greenhouse is a blendof nutrient-rich deep water and seawater (previously used forcondensation of freshwater in adjacent greenhouses). Depending on theseason, these waters can be nutrient-depleted or harbor an abundance ofconfounding but naturally occurring organisms. Running the water throughthe greenhouses permits the adjustment of water quality, for example, byadding nutrients or removing unwanted or competing biomass. This waterconditioning stage also permits a small time buffer for seawateranalysis and process adjustments to ensure that water entering the pondswill support maximum algal yield.

Conditioning the seawater in the greenhouses before it is used foraquaculture has several advantages. Naturally occurring organisms areeffectively inactivated in the heat and by the intense solar radiationof the greenhouses and thereby are prevented from out-competing thedesired algae, fouling the aquaculture ponds or stimulating the growthof coccolithovirus. Naturally occurring and inactivated biomass can beharvested before the water enters the aquaculture preserve. Measures toprevent fouling of equipment and the aquaculture ponds can be taken. Inone embodiment, water purification using peroxide can be carried outbefore the heated seawater runs through the energy extraction oraquaculture process.

Nutrients can be added to the water as it passes thorough theconditioning phase. In one embodiment, calcium minerals can be added, inparticular, in the form of apatite, to stimulate and maximize high yieldcoccolithophorid growth and prevent acidification of the seawater. In apreferred embodiment, calcium-rich minerals, nitrate and inorganicphosphate, for example, in hydroxylapatite, fluorapatite, andchlorapatite (Ca₅(PO₄)₃(OH), Ca₅(PO₄)₃F and Ca₅(PO₄)₃Cl), calciumnitrate, fertilizer known as Norwegian saltpeter (Ca(NO₃)₂),¹⁷ can beadded to enhance growth. In some embodiments, alkali minerals can beadded to directly counteract ocean acidification including CaCO₃ tore-mineralize the water, CaO (quick lime) to replace Ca directly andalkalize the seawater and ensure that the seawater reaches the samedegree of Ca super-saturation it would have had before oceanacidification.

The minerals can be added at multiple points when the seawater is pumpedor processed including in the OTEC plant, in the greenhouses while thewater is being heated, while the water is entering and passing throughthe aquaculture ponds, provided for the growth of seed algae in seedingponds that do not form part of the aquaculture ponds, added togetherwith the inoculants of algae used to seed the seawater and/or appliedafter the algae have been harvested and the seawater is returned to theocean.

Minerals can be dissolved or added as a slurry to return the seawater toa natural state of super-saturation with respect to calcium ions.¹⁸ Thealgal growth itself will accelerate the dissolution of minerals andaccelerate the absorption of minerals. This returns the seawater tonatural levels of Ca super-saturation (between 200% to 420% Casuper-saturation) and pH. It also actively mitigates oceanacidification, even after the water has been discharged from theaquaculture ponds. In many proposed coastal regions, the seawaterbrought into the OTEC plants will be of lower Ca super-saturation (dueto ocean acidification) and this method will actively improve localseawater quality.

Because of the nutrient-rich deep waters used to cool the heatexchangers in the OTEC facilities, relatively few nutrients will have tobe added to support high E. huxleyi growth rates. However,micro-nutrients and minerals may also be added to replace the Casequestered in the E. huxleyi and prevent ocean acidification and boostalgal growth. Similarly, HCO₃ ⁻ bicarbonate ions may be added to ensurethat E. huxleyi is not growth limited due to low carbon availability.

Aquaculture Ponds

Aquaculture ponds can be built from low cost materials as they onlyrequire the shaping of land without concrete retention walls or firmstructures. These coastal aquaculture ponds can be built similarly toprawn or shrimp aquaculture ponds. Highway building equipment used tosculpt the landscape before construction is sufficient. Aquacultureponds are preferably long, wide, shallow troughs with an earthen barrierthat retains the seawater. The bottom of the aquaculture ponds ispreferably lined with impermeable material. If necessary, a clay liningcan be spread underneath the water barrier to prevent saltwaterintrusion onto land. In a preferred embodiment, each aquaculture pondcan be lined with 10 cm to 20 cm thick clay lining or geomembranes usedin landfills and then covered with layers of impermeable white plasticlining or geomembrane. In addition to the clay lining, in someembodiments the aquaculture ponds also comprise a 2 mm to 5 mm thicksynthetic liner such as HDPE will be used to prevent intrusion ofseawater onto land. The advantage of synthetic liners such as HDPE isthat they are easily repaired, edges can be fused (to seal leaks) andthey are very robust. If the land needs to be repurposed, the lining canbe readily removed and the earthen barriers leveled.

The aquaculture ponds are scaled to hold about 1 to 20 days, preferably1 to 7 days, even more preferably 6 days' worth of aquaculture seawaterthat is being emitted from the OTEC-greenhouse combination plus thewater introduced from the seeding ponds. In a preferred embodiment, theaquaculture ponds are configured to accommodate the water from agreenhouse that heats 240,000 tons of seawater per day per greenhouseand an equivalent of 240,000 tons of OTEC pumped cold seawater, plus theinoculants from the seeding ponds of approximately 40,000 tons per day.This equals approximately 520,000 tons of seawater entering in the arrayof aquaculture ponds each day. For 6 days residence time in theaquaculture ponds, the total volume of an aquaculture pond in apreferred embodiment is approximately 3.1 million tons (m³) of seawater.The typical depth of a single aquaculture pond is 3 meters, whichensures maximum atmospheric gas exchange. The surface area of a typicalaquaculture pond will be approximately 1 km². In a preferred embodiment,there will be approximately 40 aquaculture ponds and four seeding pondsfor each 100 MW OTEC plant. As a person skilled in the art willappreciate, the total number and size of the aquaculture ponds will varywith the size of the OTEC plant. For example, in one embodiment, theOTEC plant is a 500 KW plant, which is provided with up to 2 km² ofaquaculture ponds and a seeding pond of a correspondingly adjusted size.In a further embodiment, a 500 MW plant is provided with approximately200 km² of aquaculture ponds, and 20 seeding ponds.

Configuration of System Components

The configuration of the OTEC plants, greenhouses, seeding ponds,aquaculture ponds and harvesting area is flexible and can be configuredto adapt to local geographic and meteorological conditions. Since watermay flow over large distances between greenhouses, pipes can be laid tomove the seawater where it is needed. These pipes may be insulated topreserve the appropriate water temperature. A typical arrangement of thecomponents is shown in FIG. 1.

In a preferred embodiment, the OTEC plant is a 100 MW plant. Typically,this size of OTEC plant can support 40 greenhouses, 4 seeding ponds forthe initial growth of the inoculant algae, and 40 aquaculture ponds witha total volume of 3.1 million m³, and associated harvesting areas.Depending on the local climate and the ambient temperature of thesurface water, each greenhouse covers on average 50,000 m².

Given that the greenhouses need to supply the OTEC plants with heatedwater, the greenhouses can be located in close proximity to the OTECplant, reducing the need to move the heated water over large distances.Since the temperature of the discharge from the OTEC plant is lesscritical before it enters the aquaculture pond, these can be transportedover larger distances without fear of heat loss.

For these reason, the aquaculture ponds can be aligned close to thecoast, hugging the coastline either in single file or arranged inparallel rows, while the greenhouses are clustered around a central OTECplant. This T-shaped arrangement will reduce pipelines needed and allowsfor the sharing of algal harvesting equipment, for example, reducingoverall capital costs.

Carbon Offsetting

Current forecasts for carbon offsets per ton of CO₂ sequestered rangefrom $7 to $170 between now and 2050.19 These estimates are based onextrapolation of the experience with the European Union Emission TradingScheme (EU ETS) that established a functioning carbon market with alarge-scale emissions cap and trade system. Between April 2005 and April2006, the spot price averaged at approximately £22 per ton carbonoffset. These prices are premised on the assumption that CO₂ emittingindustries and sources can trade their emissions for carbon offsets thatreduce equivalent emissions elsewhere. Challenges to the system haveincluded the fact that the reduction in emissions are difficult toverify and that offsetting reductions in emission would have happenedanyway (thereby reducing the value of the offsets). New proposals forCap and Trade systems are under discussion before the Copenhagen meetingin Copenhagen 2009.

The methods of carbon sequestration hereof are immune to both thesechallenges. Firstly, the sequestered carbon is physically available foranalysis and can be documented without any ambiguity. Secondly, none ofthe biomass would have been created otherwise. The purpose of the schemeis entirely to create additional biomass from abundant resources withoutoffsetting other food, biomass-generating or carbon fixing activities(and without creating new sources of CO₂ emission, for example, throughthe burning of fossil fuel to generate the power needed to operate thesystem). The carbon intensity of traditional OTEC is already lower thanthat of hydroelectric power.

Based on these advantages, the methods hereof qualify for the highestquality carbon offset ratings and can secure the maximum rewardsavailable through direct payments. Alternatively, revenue can begenerated from the biomass generated alongside the CaCO₃, making thesystem independent of the carbon offset market.

All references cited herein are incorporated in their entirety.

The disclosure will now be described in detail, with specific referenceto a system that utilizes the coccolithophorid algae Emiliana huxleyi.It will be appreciated that modification of detail can be made withoutdeparting from the scope hereof.

EXAMPLES

The preferred embodiment is based on a power plant that generates 100 MWnet power per year. The power plant is a hybrid OTEC plant combiningboth energy production using the Uehara or Kalina cycle as well asfreshwater generation through a hybrid low pressure steam generationcycle. Actual gross power production exceeds the energy production byapproximately 35% to 45%. Most of the internal consumption of power isthe result of pumping 400,000 tons of cold deep ocean water and 400,000tons of warm surface water for 24 hours per day 340 days per year.Operating the large compressors to run the Uehara or Kalina energyproduction cycle in the hybrid OTEC plant and the high volume vacuumpumps to create low pressure steam for the secondary freshwatercondensation cycle also consumes power. Given the need to move waterthrough greenhouses and pumping it over kilometers to the aquacultureponds, additional power is needed to power pumps.

Warm surface seawater will be drawn in through a series of 5 m glassfiber-reinforced plastic (FRP) pipes from a depth of 5 to 20 meters. Thewarm seawater will be distributed through a network of pipes to bedistributed across 20 greenhouses. For the 12 daylight hours, the waterwill flow over the ground of the greenhouses to a depth of 10 to 15centimeters, at a flow rate of 6 meters per minute, and across a widthof 1,000 meters for greenhouses that are 500 meters long. In thismanner, each greenhouse will warm more than 475,000 tons of seawaterfrom ambient temperature to consistently 40° C. and higher.

The surface under these greenhouses will preferably be lined with blackimpermeable plastic lining, similar to those that are used in landfills.Underneath the plastic, a clay lining or geomembrane to prevent leakswill also ensure a uniform surface for the seawater to flow over. Thegreenhouses are constructed from a plastic material spars and beams toform multiple gutter-connected tunnels of approximately 10 meters inwidth and approximately 3 to 4 meters in height. As many as 100 of these500-meter long tunnels will be connected side by side to cover as muchas 50 ha of land. Connecting the greenhouse tunnels saves material andallows for the free movement of air. The greenhouses will be coveredwith double-layered improved polyethylene (PE) with 180 μm PE UV IRouter folia and 50 μm inner folia. The inter-connected tunnels aresupported by plastic beams standing in the pool of freely flowingseawater on the ground of the greenhouses.

Exiting the greenhouses, the hot water will be collected and pumpedthrough a manifold of insulated 1 meter internal diameter pipes to filla large surge tank covered with a floating insulating material to retainthe temperature of the solar-heated water. This water is used to feedthe power-generating cycle of the OTEC power plant. The surge tankbuffers the flow of heated water as the greenhouses fill with water,after dawn when solar-heated water is available, and drains aftersunset. By doing this, they effectively extend the power generationcycle of the OTEC plant beyond daylight hours, providing processcontinuity and temperature stability.

For the condensation of both the turbine driving gas as well as thecondensation of freshwater, depending on the local ocean thermocline,the cold deep water is pumped through an array of 3 m (inside diameter)FRP reaching to a depth of 600 to 1,000 meters. On the surface, thesepipes are insulated to retain the low temperature of the deep water. Inthe hybrid OTEC plant, the cold water is used sequentially in a two-stepprocess, first to cool the power generation cycle and second, to coolthe freshwater condensers in the freshwater generation cycle.

The cold deep water discharge from the OTEC plant is nutrient rich andcollected in a cold water discharge basin to manage surges, fordistribution to the seeding ponds and to fill the aquaculture ponds. Thesurge tank again provides storage capacity to steady the flow throughthe aquaculture ponds.

Similarly, the OTEC discharge from the power generation cycle that hassignificantly cooled in the process of generating power and creatingfreshwater is collected in a warm water discharge surge basin. The waterin the warm water OTEC discharge basin is used to control the rate offilling the aquaculture ponds and control the water temperature in theaquaculture ponds as needed. This basin is also the preferred site forthe addition of minerals and nutrients to maximize the growth of algaein the aquaculture ponds. Nutrients are added as the water is dischargedfrom the OTEC power generation cycle to ensure that the residual heat,together with the residence time in the surge tank, ensures thedissolution of approximately 10 tons of the fertilizers hydroxylapatiteCa₅(PO₄)₃(OH) and 5 tons of the fertilizer Norwegian Saltpeter Ca(NO₃)₂per day. Other fertilizers may be added according to need.

The nutrient-rich cold water and the fertilized warm water are thendistributed to the seeding and aquaculture ponds. The seeding ponds areidentical to the aquaculture ponds in their structure. The onlydifference is that the concentration of algae in the ponds is lower atthe beginning, the flow rate through the aquaculture pond is slower, andthe discharge is used to seed up to 12 aquaculture ponds with algaeinstead of harvesting the algae. The purpose of the seeding ponds is toprovide consistent inoculants for the array of aquaculture ponds and toextend the effective residence time of the algae in culture. Theresidence time of E. huxleyi in the seeding ponds is 6.3 days and thisresults in an increase from 23 million cells per liter to approximately230 million cells per liter of seawater.

The water from the seeding ponds is added directly into the manifold atthe point where the cold water discharge is added to the distributionmanifold, thereby eliminating the need for a seeding pond waterdistribution system. The manifolds provide 40,000 tons of seeding pondwater, 240,000 tons of cold seawater and 237,000 tons of warm water (ora total of 477,000 tons of seawater) for each aquaculture pond per day.

From the cold and warm water discharge basins, a pipeline manifold of1-meter internal diameter pipes feeds the seeding and aquaculture pondswith mixed water to support an average flow rate of 1.1 meters perminute over a 24-hour period. The aquaculture ponds are designed toaccommodate the daily flow of 477,000 tons of mixed cold, warm andseeding pond water per day. Forty aquaculture ponds, each covering a1-kilometer square, are comprised of 100-meter wide and 3-meter deeptracks that fold back onto themselves ten times, resulting in a totalaquaculture pond length of 10,000 meters. The aquaculture ponds arelined with both a 10-cm clay geomembrane and white, double HDPE liningto prevent leaks and seawater intrusion on land.

At the flow rate of 1.1 meters per minute, the average residence time ofthe water in each aquaculture pond is 6.3 days. This residence timesupports up to ten doublings of E. huxleyi in their exponential growthphase in the aquaculture pond. With an inoculant cell density of 300,000cells per liter introduced at the beginning of the aquaculture pond, thefinal cell density at harvest is approximately 3,000,000 cells perliter. This equates to approximately 1.5 gr of coccolithophorid cellsbeing created in exponential cell growth per liter of seawater.

At a volumetric flow of 475,000 tons of water through each aquaculturepond through 40 ponds for an operational period of 340 days per year perpond, this generates approximately 10 million tons of pure CaCO₃ in theform of coccoliths per year. In addition, 150 to 180 million tons ofalgal biomass is grown to be harvested.

As the water flows through the final 100-meter section of theaquaculture pond, it passes over an array of fine-bubble curtains,generated by a series of pipes spaced 5 meters apart at the bottom ofthe aquaculture ponds. These bubbles become entrained in the algalaggregates forming naturally as the bloom of algae in the aquaculturepond matures. As the cells age, they begin to release TEP, which assistsin the cross-linking of cells into small aggregates. As bubbles becomeentrained in these aggregates, they float to the surface. Thisharvesting process can be further accelerated through the use ofclarifiers used in wastewater treatment or particulate mineralprocessing to remove residual algae. At the surface, the aggregatesaccumulate and form a thick carpet of algal slurry, which is skimmed byautomated surface skimmers. It is estimated that given the configurationof this example, each aquaculture pond will generate approximately60,000 tons of this slurry per day per pond. This surface-slurry is thenfurther thickened in deep cone thickeners or clarifiers five- toten-fold through the addition of one nanomole concentration highmolecular weight cationic polymer or flocculants. The resulting sludgeof approximately 9,000 tons of algal sludge per pond per day istransported on mesh-covered long conveyor belts where the sludge drainsmore of the remaining seawater. The aquaculture water that drained outof the algal slurry is then returned to the aquaculture ponds.

This algal paste on the conveyor belts is distributed over drying fieldsthat are lined with geomembranes to dry in the ambient air. When thesludge has reached a moisture content of approximately 40%, it can bestockpiled for further processing.

The harvested algae can be further processed by fragmentation intomultiple components. Processing of the harvested algae at a basic pHenables the separation of the coccoliths from the remaining organicbiomass.

Once extracted, the organic biomass can be used for the production ofbiological oils, fatty acids, bio-fuels,²⁰ and other materials that canbe used for the generation of, for example, animal feedstock. Furtherprocessing of the biomass can include high temperature pyrolysis at 500°C. in a fast fluid bed reactor to create combustible gas, bio-oil andbio-char. In a further embodiment, the bio-char remnants from suchfurther processing could be used to create soil conditioner orfertilizer for local agriculture. Methods for processing algae are wellknown in the art.

The seawater is gravity drained from each aquaculture pond back to theocean at a depth of approximately 10 to 60 meters or greater depthaccording to the neutral buoyancy of the water. If the pH balance at theend of the aquaculture pond requires adjusting, additional calcium-basedminerals can be added before discharge to the ocean through one- tofive-meter internal diameter pipes.

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What is claimed is:
 1. A method of sequestration of carbon dioxide fromthe atmosphere into solid form, the method comprising: culturingcoccolithophorid algae in seawater using land-based aquaculture underconditions wherein atmospheric carbon dioxide is converted by the algaeinto calcium carbonate and/or bio-char; a. wherein the seawater in whichthe algae are cultured is the discharge of ocean thermal energyconversion (OTEC) condensers and greenhouses; and b. wherein the sourcewater for the OTEC process is provided by i) cold seawater and ii)seawater that has been warmed in land-based greenhouses using solarenergy to a temperature sufficient to inactivate substantially allmicroorganisms.
 2. The method according to claim 1, wherein thedischarge of the OTEC process is combined with seawater to provide waterat the optimum temperature for coccolithophorid algal aquaculture. 3.The method according to claim 1, further comprising: adding exogenousnutrients and/or minerals to the aquaculture water.
 4. The methodaccording to claim 2, further comprising: adding exogenous nutrientsand/or minerals to the aquaculture water.
 5. The method according toclaim 1, further comprising: harvesting the coccolithophorid algae. 6.The method according to claim 2, further comprising: harvesting thecoccolithophorid algae.
 7. The method according to claim 3, furthercomprising: harvesting the coccolithophorid algae.
 8. The methodaccording to claim 4, further comprising: harvesting thecoccolithophorid algae.
 9. The method according to claim 5, wherein theharvested algae are dried and/or wherein the biomass and the coccolithsof the algae are separated.
 10. The method according to claim 6, whereinthe harvested algae are dried and/or wherein the biomass and thecoccoliths of the algae are separated.
 11. The method according to claim7, wherein the harvested algae are dried and/or wherein the biomass andthe coccoliths of the algae are separated.
 12. The method according toclaim 8, wherein the harvested algae are dried and/or wherein thebiomass and the coccoliths of the algae are separated.
 13. The methodaccording to claim 5, further comprising returning the water to the seaafter aquaculture.
 14. The method according to claim 6, furthercomprising returning the water to the sea after aquaculture.
 15. Asystem for sequestering carbon using coccolithophorid algae, the systemcomprising: a. greenhouses for heating seawater using solar energy, b.an ocean thermal energy conversion (OTEC) system adapted to use theheated surface water from step a) and cold seawater; and c. aflow-through, land-based coccolithophorid algae aquaculture preserveprovided with water from the OTEC condensers of part (b) and thegreenhouses.
 16. The system according to claim 15, wherein theaquaculture preserve comprises a flow-through aquaculture pond.
 17. Thesystem of claim 15, wherein the aquaculture preserve has a surface areaof about 1 to about 40 km².
 18. The system of claim 16, wherein theaquaculture preserve has a surface area of about 1 to about 40 km². 19.The method according to claim 1, wherein the coccolithophorid algae isEmiliana huxleyi, Gephyrocapsa oceanica, Calcidiscus leptoporus,Coccolithus braarudii, Braarudosphaera bigelowi, and/or Syracosphaerapulchra.
 20. The method according to claim 19, wherein thecoccolithophorid algae is Emiliana huxleyi.
 21. The method according toclaim 1, further comprising: adding alkali minerals to the seawater tocounteract ocean acidification.
 22. The method according to claim 2,further comprising: adding alkali minerals to the seawater to counteractocean acidification.
 23. The method according to claim 9, wherein thealgae releases Transparent Exopolymer Particles (TEP) prior toharvesting.